(1) Field of the Invention
The present invention relates to a process and an apparatus for producing a composite oxide single crystal body comprising a core portion and a clad portion in a form of a fiber or plate.
(2) Related Art Statement
Recently, single crystal fibers made of non-linear optical materials have been recognized as active members in the fields of second harmonic generation elements and optical fiber laser elements. As a process for growing an oxide single crystal, a so-called .mu. pulling down process has been noted in producing a single crystal fiber. "Densoken News" No. 522, pp 4 to 8, July 1993, Japanese patent application Laid-open No. 4-280891 and Laid-open No. 6-345,588 describe that single crystal fibers made of potassium-lithium niobate K.sub.3 Li.sub.2-2X Nb.sub.5+X O.sub.15-+X hereinafter referred to as KLN) or the like were grown by this method. It is shown that the fibrous oxide single crystals grown by the .mu. pulling down process have high structural strength and small residual stress (Jap. J. Appl. Phys. 33(1994) p 3510).
It has been desired to develop a process for forming a clad portion around a single crystal fiber having high conversion efficiency for use as a laser. For example, it is reported that a clad layer is formed around an Nd-YAG laser fiber by coating it with glass and that a clad layer having a relatively low refractive index is formed around a surface of a lithium niobate fiber by diffusion, ion exchanging or ion indiffussion implantation.
Further, several trials have been made to control a concentration profile in a single crystal in growing the single crystal from a melt. For example, a process for re-melting a single crystal is known. In this process, a CO.sub.2 laser beam is used to grow a fiber made of a single crystal ruby according to a float zone method. According to this method, the content of chromium in the surface layer of the fiber is reduced by 100 times by melting a surface layer of the fiber with a CO.sub.2 laser and re-growing this surface layer of the fiber. However, despite this treatment, it is considered that there is hardly a possibility that fiber growth is combined with re-melting of the surface layer of the fiber in one process.
Moreover, a process is available to grow a composite single crystal from two kinds of different doped melts fed. For example, a film of LiNbO.sub.3 is grown on a substrate made of LiTaO.sub.3 by a capillary liquid phase epitaxial technique (J. Crystal Growth Vol. 50 (1980) pp 291-298). However, the melts diffuse into each other to a large extent, and a transition region is as wide as several mm. Accordingly, it is impossible to realize such a steep change in refractive index as to confine the light.